Electrical connectors are commonly used that mount in holes of barrier walls to pass electrical signals through the walls while preventing the passage of fluid across the wall. One common application is in transmitting signals through a transmission wall which separates an oil-filled transmission from the environment. The connectors may have grooves on their outside that receive O-rings to seal against the surface of the hole in the barrier wall. When there is a large pressure differential across the barrier wall, it is desirable for the O-rings to press tightly against the surface of the hole to avoid leakage. However, such tight pressing of the O-rings can be undesirable in that they can cause the O-rings to become "set" wherein continual pressure on them causes a reduction in resiliency which can later lead to leakage. Also, it can be difficult to install O-rings which press tightly against the hole surface. A connector which assured tight pressure of an O-ring or other elastomeric seal when there was a high differential pressure but which avoided set of the seal, and facilitated installation of the connector, would be of considerable value.